Conventionally, disk storage devices using a head for read and write operations on a disk or the like, in a seek operation for moving the head to a target track on the disk, detect a servo pattern on which a plurality of position signals for detecting an offset position with respect to the center of the track are recorded to demodulate the position of the head. The demodulated position (decoded position) is used for control, thereby improving the response performance in the seek operation.
The position signals recorded on the servo pattern are used for obtaining an accurate decoded position during a following operation in which the head follows the track, and the position signals are not recorded thereon in consideration of the case where the head traverses the servo pattern obliquely. Therefore, in the seek operation, when a decoded position is obtained by detecting the position signals recorded on the servo pattern, an error of the decoded position increases in accordance with an increase in the velocity of the head moving in the radial direction of the disk. As a result, the velocity of the head reaches velocity (demodulation limit velocity) at which the correspondence relationship between the decoded position and the real position is obscure. The restriction by the demodulation limit velocity is one of the factors for preventing the response performance in the seek operation from being further improved.